Adding 30A Current Limiter to Low Volt-High Curret circuit

[Scopemaster]

I am quite new to electronic circuits. I have just completed my first circuit that works... WaHooo. I need to ad a couple of features and can't seem to get understanding on making it happen.

I built the 12V 30Amp PSU as found **broken link removed** . Instead of using the LM7812C I used the LM350T. Needed to regulate between 2 & 5V and needed a full 30amps.

Today I found I could adjust over 35amps. I need to limit this to fold back or at least limit the total current output. I read up on the LM350T https://www.electro-tech-online.com/custompdfs/2005/10/LM150.pdf mfg data sheet web page on page 10 explains a precision Current Limiter. I am having trouble understanding how to tie in both the by pass transistors TIP2955 & LM350T to the discribed current regulation.

And as a second request: Is there a method to hook up a LED to indicate issues such as over current conditions should this happen?[[/url]

 

[Nigel Goodwin]

Re: Adding 30A Current Limiter to Low Volt-High Curret circu

I am quite new to electronic circuits. I have just completed my first circuit that works... WaHooo. I need to ad a couple of features and can't seem to get understanding on making it happen.

I built the 12V 30Amp PSU as found **broken link removed** . Instead of using the LM7812C I used the LM350T. Needed to regulate between 2 & 5V and needed a full 30amps.

Today I found I could adjust over 35amps. I need to limit this to fold back or at least limit the total current output. I read up on the LM350T https://www.electro-tech-online.com/custompdfs/2005/10/LM150-1.pdf mfg data sheet web page on page 10 explains a precision Current Limiter. I am having trouble understanding how to tie in both the by pass transistors TIP2955 & LM350T to the discribed current regulation.

And as a second request: Is there a method to hook up a LED to indicate issues such as over current conditions should this happen?[[/url]

The addition of the bypass transistors prevents the internal current limiting working, so it's of no use.

The usual way is to monitor the voltage drop across one of the emitter resistors - however, if it's current limiting the dissipation in the transistors is going to increase greatly.

What exactly are you trying to do?, and why do you desire current limiting?.

 

[Scopemaster]

The power source will be from a typical vehicle 12V battery system. I have a load of which I need to limit to 2.5 - 3.5 volts. In additon the current draw from the load can at times exceed 60 amps.

I can manually adjust the load to only accept around 30-40 amps. But as temperature increases the current also increases. Then there are other factors involved which can cause the current to get out of control.

I need to find a way to limit the current to 30 amps to facilitate electronic control from a run away situation under various conditions.

I am also thinking a different approach to reducing the voltage from 12V to 2.5/3.5V will be required for current controlling. Is it possible to add on a current limiter to a circuit as shown on pdf page 12 by adding 6 additional LM350's to the circuit as shown on https://www.electro-tech-online.com/custompdfs/2005/10/LM150-2.pdf.

Or it there still yet a better way to get to where I need to be?

 

[Oznog]

I would definitely use a switching regulator (buck converter) instead of a linear reg for 30 amps. It won't generate much heat at all and will draw much, much less current from the 12v battery.

 

[panic mode]

at 2.5V and 40Amp load your existing regulator burns about 400 Watts
and no wonder it gets so hot. with switcher, you shouldbe able to reduce the heat disipation to some 50-60W.
i agree with Oznog's recomendation. if you make switching regulator, input current will be much lower (up to some 10A)
even though the output current is some 40A.

 

[Scopemaster]

Have any suggestions where I may locate a schematic so I can study it? I am quite new to this and can see that I will be messing aropund for quite some time. Unfortunately official schooling is out of the question for this year so the internet needs to be my primary source.

Have any good suggestions on a good switching regulator and a diagram on the best possible way to set up an adjustable voltage and current limiting at the low volts x high current?

 

[Ldi Edt]

I would suggest building a synchronous buck converter.

Look at TI, ON Semi, National and the like for app notes.

Keep up the good work. Evaluate each approach for pro's and cons, and soon enough you will be designing your own circuits.

 

[Optikon]

Have any suggestions where I may locate a schematic so I can study it? I am quite new to this and can see that I will be messing aropund for quite some time. Unfortunately official schooling is out of the question for this year so the internet needs to be my primary source.

Have any good suggestions on a good switching regulator and a diagram on the best possible way to set up an adjustable voltage and current limiting at the low volts x high current?

For possibly a more educational experience, see if you can use linear tech controllers (dont get a switching regulator with the switch built in, you wont find one that can do 30+ amps). The controller, allows you to place an external mosfet of your choice (picking one that can do 30+ A).

After you've worked out the design, you can use Linear tech simulation software to actually simulate the circuit with a load to see if it works before you spend effort building the design. It will let you evaluate whether or not you circuit will overheat and lets you calculate power efficiency & play what if games like your 12V supply being more like 14.5V etc...

 

[panic mode]

this might be good read...
https://members.tripod.com/michaelgellis/power4.html

 

[Oznog]

There's a bunch of switching regulator chips that for $0.75-$5. They usually have some sort of current limiting solution integrated into them.

If the switching losses can be managed, there's some 8-SOIC pkgs that can handle that much power with one or two packages. MOSFETs parallel easily.

Most designs you'll see show a Schottky diode rather than sychronous rectifiers. Schottky is much simpler, but 30 amps requires a pretty big one that makes a lot of heat. Synch rects will generally require you find a chip designed to support it.

Inductor selection is the part that requires some though. You need to stay far from the saturation limit, thermal limit due to i^2*r losses, and enough inductance to meet your ripple specs.

In the end you'll have a device that only makes a few watts of heat at most. If designed carefully it may not even require a heat sink.